(Image inside) UV offcenter but textures working. Causing issues
The textures render fine because they are "repeated" by default: you can display this behaviour by clicking the relative option in the Properties panel (N) of a UV editor window.

In my example, as the image is 2048x2048, if I select all UV vertices and grab (G) them by 2048 in the Y axis I would get your situation: the render would be exactly the same.

In your case you can read the Y dimension of your texture (let's say 1024 x 1024), select all the UV and press G,Y,-,1,0,2,4,Enter.

The UV will be centered, the render will be the same and Unity will be happier.

I don't know why the modeler did this little mistake.